Most of the lead acid batteries manufactured today use plastic, microporous, silica filled separators between the positive and negative plates to provide a means for preventing shorting between adjacent plates of opposite polarity, as a means for allowing electrolyte flow against the plate surfaces and as a means for maintaining the relative spacing between the plates and the battery box.
A frequent method in using these separators is to form them as envelopes around one or both of the plates (positive and negative). To form an envelope, one may simply cut a piece of separator sheet to a length about twice that of the electrode plate, place the electrode plate on one half of the sheet and fold the other half of the sheet over the other side of the electrode plate. The edges of the sheet perpendicular to the crease are typically sealed by mechanical crimping, adhesives or heat welding.
The enveloped plates are then arranged with other plates of opposite polarity (which may be enveloped, but typically are not) into a battery box. Electrolyte (typically sulphuric acid) is added and the box is sealed to complete the battery.
Enveloping is desirable for several reasons. It allows for automation of the separator and electrode insertion. It forms a complete physical barrier between adjacent plates of opposite polarity so as to prevent dendritic growth which leads to shorting. When the positive plate is enveloped, it prevents the sloughing off of active material and eliminates the need for mud racks which kept the plates out of the sloughed off material of the older "interleafed" type of battery.
While enveloping has a number of advantages, it still has drawbacks. Perhaps the largest drawback is oxidation resistance. The formation of a crease along the bottom (if one piece of separator is used) and sealed edges (either along the vertical side edges and/or optionally the side edges and the bottom edge when two pieces are used to form the envelope), provides an area which is more susceptible than other portions on the envelope to oxidation by the components of the battery. The formation of cracks, tears, etc. tend to concentrate in these areas. Over time they can lead to dendritic growth and other problems which shorten battery life.